1633 Design Basis for FRP Strengthening Systems

A design basis for FRP strengthening systems for concrete structures has been recommended by a number of standards and professional organizations. The LRFD basis is recommended by the ACI 440.2R-02 (2002); however, at this time resistance factors are not probabilistically based. Load factors are those recommended for all concrete structures by ACI 318-99.

For the design of concrete members with FRP strengthening systems the ACI recommends the following resistance factors:

Characteristic strength (also called the guaranteed strength) and strain to failure of FRP composite materials for strengthening are defined as the mean minus three standard deviations of a minimum of 20 test samples. The design strength, fu, and design failure strain, efu, are obtained from the characteristic strength and failure strain by multiplying them by an environmental reduction factor, CE, which depends on the fiber type in the FRP strengthening system and the type of intended service of the structure. For example, for weather exposed concrete with a glass-reinforced epoxy FRP strengthening system, CE is 0.65 (ACI 440.2R-02 2002).

While flexural strengthening over 100% of the original strength is achievable, the ACI limits the amount of strengthening to prevent catastrophic failure of the concrete member in the event of loss of, or damage to, the strengthening system (due to vandalism or environmental degradation). The ACI recommends that the strengthened member have sufficient original factored capacity (i.e., discounting the additional strengthening system) to resist a substantial portion of the factored load on the strengthened member, given as

To account for fire, additional restrictions are placed on the factored capacity of the FRP strengthened structured (ACI 440.2R-02 2002).

When a concrete member is strengthened to increase its capacity in a selected mode (e.g., flexure) the member must be checked to ensure that the capacities in other failure modes (e.g., shear) are not exceeded. If this is the case the strengthening should be decreased or the secondary capacity needs to be enhanced with its own strengthening system.

At this time the ACI 440.2R-02 guide does not provide recommendations for the determination of serviceability criteria (such as deflections or crack widths) for FRP strengthened members. The ACI 440.2R-02 guide does, however, limit the stress in the steel at service loads to 80% of the steel yield stress and limits the ''sustained plus cyclic'' stress in the FRP strengthening system to account for creep rupture and fatigue depending on the fiber system. For carbon FRP strengthening systems this limit is 55% of the ultimate strength. For deflections in flexural members where stresses are in the service range the contribution of the FRP strengthening system is typically small. Flexural deflections in the service range can be estimated by the use of an effective ''composite-section'' second moment-of-area (Ie) analysis where the tensile contribution of the FRP is added to the contribution of the steel reinforcing. In the inelastic range (after the primary reinforcing steel has yielded) the contribution of the FRP strengthening to the postyield stiffness can be quite considerable and should be accounted for, in inelastic analysis.

It is extremely important to note that the method of determining the tensile force resultant in an FRP strengthening system depends on the type of system used. In the bonded strip (or plate) system the ultimate force is obtained from the strength of the FRP composite (see Table 16.5) and the gross cross-sectional area of the strip. In the dry fiber systems the ultimate force is obtained from the strength of the fibers and the thickness of the net area of the fibers (see Table 16.6). The designer must know if the reported strength (and stiffness) for an FRP strengthening system is for the FRP composite (gross cross-section) or for the fibers alone (net fiber cross-section). Both methods of calculation are permitted by the ACI 440.2R-02 guide at this time.